1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a data driving apparatus and method for a liquid crystal display device.
2. Discussion of the Related Art
A typical liquid crystal display (LCD) device includes a timing controller for outputting various control signals to control the driving of a gate driving unit and a data driving unit. The gate driving unit is for applying a gate on signal to each gate line on a liquid crystal panel, and the data driving unit is for applying a data signal to each data line on the liquid crystal panel. The liquid crystal panel is driven by the data signals and the gate on signals to display images.
The timing controller uses longitudinal/horizontal synchronization signal and clock signal received from a system to generate a gate control signal for controlling the gate driving unit and a data control signal for controlling the data driving unit. Additionally, the timing controller samples digital video data (RGB) input from the system and rearranges those sampled data to send to the data driving unit.
The gate driving unit responds to the gate control signal received by the timing controller by sequentially sending a scan pulse to each of gate lines GL1˜GLn. Accordingly, horizontal lines on the liquid crystal panel are selected.
The data driving unit responds to the data control signal input to the timing controller to convert the digital video signal (RGB) into pixel signals corresponding to a grayscale value. The converted pixel signals are accordingly sent to each of data lines DL1˜DLm on the liquid crystal panel.
The liquid crystal panel includes a plurality of liquid crystal cells arranged in a matrix on crossings between the data lines DL1˜DLm and the gate lines GL1˜GLn. The plurality of liquid crystal cells are driven by the pixel signals and the gate on signals to allow images to be displayed on the liquid crystal panel.
In order to drive the liquid crystal cells on the liquid crystal panel in the LCD device, inversion systems, such as a frame inversion system, a line inversion system and a dot inversion system, are used. The frame inversion system inverts a polarity of a pixel signal applied to each of the liquid crystal cells on the liquid crystal panel whenever a frame is changed. The line inversion system inverts a polarity of a pixel signal applied to each of the liquid crystal cells depending on lines (columns) on the liquid crystal panel. The dot inversion system is configured such that the liquid crystal cells on the liquid crystal panel are provided with pixel signals with opposite polarities to the pixel signals applied to the liquid crystal cells adjacent thereto in horizontal and vertical directions, and in addition the polarities of the pixel signals applied to all the liquid crystal cells on the liquid crystal panel are inverted for each frame. The dot inversion system can provide images with greater quality than the frame inversion system and the line inversion system. The inversion systems are driven using a data driving unit that responds to a polarity inversion signal applied from the timing controller.
An LCD device may be driven at a frame frequency of 60 Hz. However, in a system requiring low power consumption such as a notebook computer, the frame frequency may be decreased down to 50˜30 Hz. As the frame frequency is decreased, flicker may occur in the dot inversion system. Accordingly, a two-dot inversion system as shown in FIGS. 1A and 1B is widely used.
FIGS. 1A and 1B show polarities of pixel signals for an odd frame and an even frame, the pixel signals supplied to a liquid crystal panel driven by a two-dot inversion system. It can be seen in FIGS. 1A and 1B that the polarities of the pixel signals are inverted on a dot unit basis in a horizontal direction as in the existent dot inversion system, while they are inverted on a two-dot unit basis in a longitudinal direction. The two-dot inversion system can effectively reduce flicker on a commercial screen driven at a frame frequency of 50 Hz as compared to the dot inversion system. However, the two-dot inversion system may generate a problem of a stripe phenomenon horizontally generated as explained hereinafter with reference to FIGS. 2 and 3.
FIG. 2 shows pixel signals and waveforms of charging voltages applied to liquid crystal cells on four arbitrary scan lines adjacent to one another during a certain frame on a liquid crystal panel driven by the two-dot inversion system as shown in FIGS. 1A and 1B. For a pair of positive pixel signals or a pair of negative pixel signals consecutive as a pair in a longitudinal direction sequentially supplied to the four arbitrary scan lines, the pixel signals {circle around (1)} and {circle around (3)} applied to odd-numbered scan lines rise or fall relatively slowly to reach their target levels rather than immediately transiting to the highest or the lowest level. On the other hand, the pixel signals {circle around (2)} and {circle around (4)} applied to even-numbered scan lines are transition rapidly to their target levels.
The difference in transition time occurs because for the pixel signals {circle around (1)} and {circle around (3)} applied to the odd-numbered scan lines of the positive pixel signals or negative pixel signals consecutive as the pair in the longitudinal direction sequentially applied to the four arbitrary scan lines, a relatively long rising or falling time is needed when the pixel signals are changed from positive signals to negative signals or vice versa, while for the pixel signals {circle around (2)} and {circle around (4)} applied to the even-numbered scan lines, such a time is not needed because the pixel signals are changed from signals with the same polarity.
As a result, among pixels corresponding to the four scan lines to which consecutive positive pixel signals or consecutive negative pixel signals with being adjacent to each other in the longitudinal direction are applied, the even-numbered pixels are fully charged to a level very nearly equal to the target level, but the odd-numbered pixels are not fully charged to the target level. For a liquid crystal panel with high resolution, the available time becomes short for gate signals and pixel signals, and in particular, delay is more severe for the pixel signals. Accordingly, the pixel signals on the odd-numbered pixels may have worse charging characteristics resulting in a visible occurrence of horizontal stripe phenomenon as shown in FIG. 3.
In the two-dot inversion system of the LCD device of the related art, the even-numbered pixels among the longitudinally adjacent four pixels are fully charged up to the almost desired levels but the odd-numbered pixels are not fully charged, thereby causing the horizontal stripe phenomenon due to the difference of brightness.